The invention relates to recovery of hydrocarbon vapors from fuel storage tanks.
Vacuum-assisted Stage II vapor recovery systems serve to recover hydrocarbon vapors displaced from vehicle fuel tanks during fuel dispensing. Present systems have a tendency to return a greater volume of air and hydrocarbon vapor to the underground gasoline storage tanks than the volume of liquid gasoline dispensed to motor vehicles. This positive volumetric imbalance is partially accommodated by using pressure/vacuum vent valves on the tank vents to allow pressure build-up within the tank ullage space without emission of hydrocarbon vapors from the tank vent, relying on condensation of the hydrocarbon vapor to relieve the pressure over time.
However, since the standard leakage test for service station ullage space permits a leakage rate of up to 0.1 cubic feet of air per minute at a positive pressure of 5-inch water column (WC), any device or system which tends to cause positive pressure will result in emission of hydrocarbon vapor through existing system leaks, e.g. at pipe joints.